Field of the Invention
The invention relates to a pressure relief valve for a hydraulic system as well as a hydraulic system for controlling a belt-driven conical-pulley transmission (CVT). The invention also relates to a belt-driven conical-pulley transmission controlled thereby and to a motor vehicle equipped therewith.
Description of the Related Art
Belt-driven conical-pulley transmissions can have a continuously variable transmission ratio, in particular automatically occurring transmission ratio variation.
Such continuously variable transmissions include, for example, a start-up unit, a reversing planetary gearbox as the forward/reverse drive unit, a hydraulic pump, a variable speed drive unit, an intermediate shaft, and a differential. The variable speed drive unit includes two conical pulleys and an encircling element. Each conical pulley includes a first, axially fixed conical disk and a second, axially displaceable conical disk. An endless torque-transmitting means runs between the pairs of conical pulleys and can be, for example, a steel thrust belt, a traction chain, or a belt. By axially displacing the second conical disk relative to the first conical disk, the running radius of the endless torque-transmitting means changes, and hence so does the transmission ratio of the continuously variable automatic transmission.
Continuously variable automatic transmissions require a particular pressure level, sometimes high, in order to be able to move the conical disks of the variable speed drive unit with the desired speed at all operating points, and also to transmit the torque with sufficient basic pressing force largely without wear. The overall control can be provided by means of an electrical control system, which can include electrically operated proportional valves, for example.
An object of the present invention is to provide an improved pressure relief valve, in particular one that is less hysteresis-prone, simpler to produce, and/or more temperature stable.